Various techniques for producing 3-D holographic images are well known but a brief explanation is included for the sake of completeness and for assisting in the reader's understanding of the present invention. Briefly, an object is illuminated by an object beam of coherent light from a laser source. An image recording medium, commonly a photographic plate is exposed to the object light reflected from the object. Simultaneously, a reference beam derived from the source, is directed on to the surface of the photographic plate. By careful arrangement of the apparatus, the coincidence of the object beam and the reference beam generates a pattern of interference fringes which record the intensity and phase of the wavefront of the object light. By developing the plate and illuminating the resulting interferogram using a reconstruction beam similar to the reference beam, a virtual or a real image of the object can be created.
Examples of a hologram recording apparatus and a hologram reading apparatus are disclosed in U.S. Pat. No. 7,940,438, issued on May 10, 2011, which is incorporated herein by reference in its entirety. For the sake of completeness, a portion of this patent is described below with reference to FIGS. 1, 2 and 3.
FIG. 1 is a diagram showing a hologram recording/reading (reproducing) apparatus 1. As shown, the hologram recording/reading apparatus 1 includes a light source 10, a shutter 12, a half wave plate 14, a polarizing plate 16, an enlarging/collimating optical system 18, a mirror 20, a polarization beam splitter 22, a spatial light modulator 24, lenses 26, 28 and Fourier transform lenses 30, 32 constituting a relay lens system. Also included are a filter 34, a Fourier transform lens 36 for focusing the light in a hologram recording medium 100, and a Fourier transform lens 38 for relaying transmitted light (reproduction light) transmitted through the hologram recording medium. Further included are a medium holding portion 40 for holding the hologram recording medium 100, Fourier transform lenses 42, 44 constituting a relay lens system, a filter 60 and a light receiving element 50.
The light source 10 irradiates coherent light acting as a light source for the signal light and the reference light. As coherent light, a light source such as a laser beam may be employed. As a laser beam, a wavelength (for example, a green laser) of 532 nm may be employed.
The laser beam passes through shutter 12, and through the half wave plate 14 and the polarizing plate. The laser beam, after being collimated, enters into polarization beam splitter 22. The polarization beam splitter 22 transmits p-polarized light of the incident laser beam and reflects s-polarized light. The laser beam reflected by the polarization beam splitter 22 enters into the spatial light modulator 24.
The spatial light modulator 24 polarizes and modulates the laser beam, in accordance with a pattern of recording information. The recording information is represented by a pattern image of bright and dark, in which digital data “0”, “1” corresponds to “bright”, “dark”, respectively.
FIG. 2 shows an example of a configuration of the spatial light modulator 24. As shown, the spatial light modulator 24 is arranged to include a reference light pixel area 200 for modulating the reference light, and a signal light pixel area 300 for modulating the signal light. The signal light pixel area 300 is disposed at the center portion and the reference light pixel area 200 is disposed at the outer periphery.
Each of the reference light pixel area 200 and the signal light pixel area 300 is configured by a plurality of pixels and each of the pixels is intensity-modulated into bright or dark patterns. The painted pattern representing the “dark” pixels is differentiated between the reference light pixel area 200 and the signal light pixel area 300 merely for the sake of the explanation, and actually each of the color and pattern of the “dark” pixels is not differentiated.
The pixels contained in the signal light pixel area 300 generate a two-dimensional (2-D) image obtained by coding page data to be recorded and subjects the signal light to the spatial modulation. Also, the reference light pixel area 200 may generate a two-dimensional image obtained by coding a random pattern and subjects the reference light to the spatial modulation.
The spatial light modulator 24, as an example, is characterized in that the pitch of the pixels contained in the reference light pixel area 200 differs from the pitch of the pixels contained in the signal light pixel area 300. That is, the pitch of the pixels contained in the reference light pixel area 200 is d1 and the pitch of the pixels contained in the signal light pixel area 300 is d2; d1 is smaller than d2.
Recording light, including the signal light and the reference light, which is subjected to the spatial modulation by the spatial light modulator 24, is relayed by lenses 26, 28 and entered into the Fourier transform lens 30. The recording light is focused by the Fourier transform lens 30 and passed through filter 34. A frequency band of the recording light is cut when passing through filter 34.
The recording light, which is passed through filter 34, is converted into collimated light again by the Fourier transform lens 32 and entered into the Fourier transform lens 36 for focusing the recording beam in the hologram recording medium 100.
The hologram recording medium 100, which is held by the medium holding portion 40, forms a hologram (interference fringe) from the interference between the reference light and the signal light.
Next, when reading or reproducing the hologram, only the reference light is irradiated upon the hologram recording medium 100. The irradiated reference light is diffracted by the hologram and so reproduction light is obtained. The reproduction light obtained includes the reference light and the signal light irradiated at the time of forming the hologram.
FIG. 3 shows an example of the focal plane of the Fourier transform lens 42 disposed at filter 60. As shown, a 0-order DC component 400 of the signal light is located at the center of the Fourier transform plane, and primary-order DC components 410 of the signal light are located around the 0-order DC component. Further, primary-order DC components 510 of the reference light are located on the outside of the primary-order DC component of the signal light. In the example, a spot distance L1 of the reference light can be represented by Expression (1) and a spot distance L2 of the signal light can be represented by Expression (2), where d1 represents the pitch of the pixels of the reference light pixel area 200 and d2 represents the pitch of the pixels of the signal light pixel area 300 in spatial light modulator 24, f represents the focal distance of the lens, and λ represents the wavelength of the coherent light.L1=fλ/d1  (1)L2=fλ/d2  (2)
When filter 60 is configured as a low pass filter having a transmission portion with a radius r satisfying the relation of L2<r<L1 and is disposed at the Fourier transform plane, as shown in FIG. 3, the reference light can be cut from the reproduction light and only the signal light having the desired frequency band can be transmitted and extracted.
Having described examples of hologram recording and reproducing apparatus, including examples of a spatial light modulator and a Fourier image, the present invention will now be described below.